The present invention relates generally to a rain sensor for detecting rain conditions for controlling operation of a windshield wiper depending upon detected rain conditions. More specifically, the invention relates to a rain sensor having a vibrator vibrating in accordance with rain conditions and with a resonating frequency in a specific frequency range which is out of the range of possible vehicle body vibrations.
In order to control wiper operation, especially to control intermittent wiper operation, rain condition dependent wiper control systems have been proposed and developed. Such rain condition dependent wiper control systems require a rain sensor which detects rain conditions to produce a sensor signal representative of the detected rain conditions. In such rain sensors, a vibration sensor has been used to detect the nature of vibration of a vibrator element onto which rain drops impact. The vibration sensitive rain sensor, however, tends to produce sensor signals in response to vibrations of the vibrator due to vehicle body vibrations caused by engine vibration, road shock and so forth. Since the wiper control system is arranged to respond to the sensor signal to determine the wiper operation depending upon the sensor signal value, the wiper may be driven even when rain is not falling.
In other words, in order to control wiper operation in accurate correspondence to rain conditions, it is necessary to provide a rain sensor which can avoid the influence of vibrations transmitted through the vehicle body. Experimental evidence reveals that the highest possible vibration frequency transmitted through the vehicle body is approximately 9 KHz. Therefore, by realizing a resonant frequency higher than the possible highest vibration frequency, the influence of vibration noises on detection of rain conditions can be satisfactorily avoided. The resonant frequency increases as the thickness of the vibrator increases, while vibration amplitude due to impact of rain drops would decrease. On the other hand, the resonant frequency decreases as the ratio of the thickness of the vibrator to the area thereof decreases, while the vibration amplitude increases.
Therefore, in order to obtain a sufficiently high resonant frequency, the thickness of the vibrator has to be thick enough in relation to the area thereof. On the other hand, in order to ensure sufficient sensitivity of the rain sensor in response to rain drop impact, the vibration amplitude of the vibrator has to be great enough for the vibrator to vibrate even in response to the relatively small impact energy available under relatively light rain conditions.